Process for obtaining bright zinc coating



alkaline zinc plating solution.

Patented Dec. 7,. 1937 mm i. Henric'ks,

diana.

lite Company, Detroit, Mich, a corporation of I No Drawing. Application May 2, 1936,

. 7 Serial N0. 77,527

in Claims. (01. 204-18) i This invention relates to electrodeposition of zinc, particularlyfrom an alkaline bath which has, or can be given, a bright lustrous appearance and to a composition of matter for forming such It has been found previously; that good zinc plates can be obtained only wh'enthe purity of the solution and the anodes are controlled. Such plates canbe bright dipped in solutions of chromic acid, United States Patent 2,021,592, to

George Dubpernell and Karl Gustaf Soderberg and application Serial'No. 45,416 to the same inventors acidifled hydrogen peroxide application Serial No. 10,086 to Karl Gustaf Soderbe'rg, bro- .mic andiallied acids, Serial No. 21,276 to David B. Stockton, anddilute nitric acid, United States ture is then allowed Patent 1,816,837 to Carl L. Ganser. However, the plates obtained in this mannerare' not as lustrous as is sometimes desired; Thus, one object of this invention is to produce zinc plates whichhave a mirror-like lustre.

Ithas been found that plain zinc cyanide solutions are rather sensitive to impurities. Thus another object of th'is invention is to produce zinc plating solutions which will tolerate higher amounts 'of impurities, particularly lead and produce bright or bright dip cadmium, and still pable plates. w I

I have found that.these desirablejresults can be had by the addition to the zinc plating baths small amounts of ammonium thiocyanate-formaldehyde resins.

M These ammonium thiocyanate-formaldehyde:, resins can be manufactured according to any of" the processes heretofore known inthe art or 'according to the following preferred methods which I have originated. w

Twenty-five pounds of technical ammonium thiocyanate containinga minimum of 90% of NH4CNSis placed in a 30 gallon steel tank. 4% gallons of formaldehyde water solution, El technical, is carefully'added, with an exhaust-fan running, to minimizeobjectionable fumes; then is addedl lbs. of caustic soda. technical, and 2% lbs. of cresol, technical, and the mixture is thoroughly stirred with a wooden paddle. It is then boiled-for 20 minutes withintermittentstir ring. to keep the brightener from precipitating too heavily around the heater coils'. The mixw to cool'and when the super-& natant liquor has clarified it is decanted into as convenientreceptacle and the remaining bright-1 ener is allowed to dry and harden. It can then be removed fromthe tank and pulverized. The decanted liquor contains. considerable amount of dissolvedbrightener. It may be used either together with the next batch or the dissolved resin may be precipitated by means of slowneutral ization with an acid such as dilute sulphuric acid.

half pound tr tartaric acid tent 'of about.85%

The total yield is approximately 30 lbs. of brightener. 1

Purer chemicals may be used in the condensa- Jtlon butthe' ordinary technical grades are fully satisfactory and cheaper.

The function of the causticl soda in the above reaction is'that of a catalyst. Any alkaline salt can be used for this purpose suchas caustic potash, sodium and potassium carbonate, sodium and potassium sulphocy'anate, cyanide of sodium or potassium, or sodium or potassium acetate, or

, other alkali'salts of organic acids. The amounts requiredare proportional to the molecular weight of thesalt. n w One can also use an acid catalyst such as one- Proportional amountsflof citric-"oxalic, acetic, furoic, may also be used with equalsuccess, also small amounts of mineral acids. i 1 i The cresol is used for a 'solubilizer- ,It not only makes the resin more easily dispersiblein the. plating solution, but it also prevents the precipl-f tation of insoluble zinccompounds with the resin.

Inlts absence, the resin isslowly removed from the bath and gradually loses its effect. Among other si'milarcompounds which have proven satisfactory are-phenol and phenol-sulphonic acids, quinol; naphth'ol'and naphthol sulphonic acids.

'The resinmay also be solubilized in othermanners. I One gram .of the resin may be dissolved in 1 0,rnl.'of 66 ,B. sulphuric acid at room temperature. This mixtureispoured into a solution containing grams of sodium carbonate in 150. m liioi Water, which neutralizes the excess sulphuric acid, This v gives a flocculent, precip tate of asulphonated resin, which dissolves to the exgivesgood plate, although the lustre has been lessened slightly ,by'the sulphonation.

Ten grams of the resin may be dissolved in 25 ml. of a solution containing 5 grams of urea, and the mixture boiled for 20, minutes. This gives a clear, yellow syrup which is completely dispers ible in the zinc bath and which gives a lustrous deposit.

Another preferred formula comprises sang/1.

on boiling. Sulphonated resin of technical ammonium thiocyanate, 750 ml. of

37% of technical water solution of formaldehyde,

.7,5 ;gramsof tartaric acid. These three composiax. ture allowedto cool down to F. The mixture is kept at this temperaturei15il F.) for 20 tions are mixed and brought to boiling temperaminutes. About 400 grams-of yellow resin precipitates and can be separated by dec'antationi This resin whenadded to the zinc plating baths] suchas zinc cyanide plating baths, gave brilliant plates," the concentrations ranging fromn6 to .9 gram per liter. However, this resin was not solubilized, and althoughit ,worked very well forthr or four days, at the end of such time part of the resin had precipitated and fallen to the bottom of the bath thereby requiring the addition of more resin to bring the bath into optimum operating condition.

In testing the brightening effect of these resins, I used a zinc solution containing 4.5 oz./gal. of zinc, 9 oz./gal. of total cyanide, expressed as sodium cyanide, and 10 oz./gal. of total caustic soda. This bath had been prepared from a pharmaceutical grade of zinc oxide and good technical grades of sodium cyanide and caustic soda. In making a new zinc plating solution, preferably the ammonium thiocyanate formaldehyde resin, zinc salt (zinc cyanide or zinc oxide), caustic soda and sodium cyanide are mechanically mixed and this mechanical mixture is then dissolved in water to form the plating solution. The mechanical mixture can be, and preferably is, dissolved in water previously run into the plating tank. In preparing a new plating solution this mechanical mixing of the resin, zinc salts, sodium cyanide and caustic soda, prior to dissolving the same in water, is considerably more efiicient and time saving than separately adding to, and dissolving in, the water each of the above-mentioned constituents of the new plating bath. Of course, as the resin, zinc salts, sodium cyanide and caustic soda are depleted by operation of the bath, each is separately added to the plating solution from time to time as required. The anodes employed were at least 99.97% pure zinc. Less pure grades may be employed if the solution is purified subsequent to make up and if the anodes are alloyed with mercury and such low current densities are applied that the mercury only amalgamates with the impurities and drops to the bottom of the tank and does not go into solution.

In the above bath we find that the optimum concentration of the standard resin is between .5 and 1.0 g/l. Bright plates are obtained at 10 amp/sq. ft. and very brilliant plates at higher current densities. At lower concentrations of brightener the plates tend to be streaky at low current densities; and at higher current densities the plates are not very bright, although there is a great deal of grain refinement. At higher conccntrations only the low current density plates are bright while the plates obtained at higher current densities are ribbed or burned on the edges.

In general the solubility of ammonium thiocyanate formaldehyde resin in the plating bath is not critical. However, the brightening effect of the resin decreases with increasing solubility and vice versa the brightening effect of the resin increases with decreasing solubility of the resin. In other words, the greater the solubility of the resin the less brightening effect and the lesser the solubility of the resin the greater the brightening effect.

The use ofv larger amounts of solubilizer in preparing the resin makes it necessary to use higher concentration of resin in the bath. Slightly different optimum concentrations are found when different catalysts or solubilizers are employed,

but generally the optimum concentrations vary between .1 and 2.5 g/l.

As stated before, the low current density plates are generally not as bright as the high current density plates, consequently the brightness of a deeply recessed part varies from the high current density points to the low current density points. If the concentration of brightener is too low, the lustre may not always be as high as desired and the plate may have a slightly brownish cast. 1 These difficulties are overcome by the use of bright dips, as mentioned above, whereby the lustre is equalized or increased. I

When the optimum amount of resin has been added to the zinc bath, its tolerance to copper is increased about six times; to cadmium more than three times; to lead, at least times.

In general, the required concentration of brightener increases with increasing metal and caustic soda content in the bath and decreases with decreasing sodium cyanide content.

The substitution of other aldehydes, such as acetaldol and furfural, for formaldehyde produced resins which did not give any brightening effect. It is evident that the high lustreobtained with formaldehyde is specific.

I claim:

1. The method of electrodepositing zinc, comprising electrodepositing the said metal from a cyanide plating solution containing an ammonium thiocyanate formaldehyde resin which is soluble in the cyanide plating solution.

2. The method of electrodepositing zinc, comprising electrodepositing the said metal from a cyanide plating solution containing a solubilized ammonium thiocyanate formaldehyde resin.

3. The method of electrodepositing zinc, comprising electrodepositing said metal from a cyanide plating solution containing an ammonium thiocyanate formaldehyde resin formed by reacting ammonium thiocyanate and formaldehyde in the presence of a catalyst in the form of an alkaline salt.

4. The method of electrodepositing zinc, comprising electrodepositing said metal from a cyanide plating solution containing an ammonium thiocyanate formaldehyde resin formed by reacting ammonium thiocyanate and formaldehyde in the presence of a catalyst in the form of an acid.

5. The method of electrodepositing zinc, comprising electrodepositing the said metal from a plating bath consisting of a relatively pure cyanide solution and containing an ammonium thiocyanate formaldehyde resin which is soluble in the cyanide plating solution.

6. The method of electrodepositing zinc, comprising electrodepositing the said metal from a plating bath consisting of a relatively pure cyanide solution and containing an ammonium thiocyanate formaldehyde resin which is soluble in the plating bath and in the presence of a pure zinc anode.

7. A plating bath for electrodepositing zinc, comprising a cyanide solution of said metal and an ammonium thiocyanate formaldehyde resin soluble in the said cyanide solution.

8. A composition of matter comprising a me- 1 chanical mixture of zinc oxide, sodium cyanide,

caustic soda, and ammonium thiocyanate formaldehyde resin.

9. A compositionof matter comprising a mechanical mixture of zinc cyanide, caustic soda, sodium cyanide and ammonium thiocyanate formaldehyde resin.

10. A composition of matter comprising a mechanical mixture of a zinc salt, caustic soda, sodium cyanide and ammonium thiocyanate formaldehyde resin.

.JOHN A. HENRICKS. 

